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Operando X-Ray Absorption Spectroscopy of WO3 Photoanodes

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Operando X-Ray Absorption Spectroscopy of WO3 Photoanodes Operando X-ray absorption spectroscopy of WO3 photoanodes Martina Fracchia,1 Vito Cristino,2 Alberto Vertova,3,4 Sandra Rondinini,3,4 Stefano Caramori,2 Paolo Ghigna,1,4 Alessandro Minguzzi*,3,4 1 Dipartimento di Chimica, Università degli Studi di Pavia, Viale Taramelli 13, 27100 Pavia, Italy 2 Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Ferrara, Via Luigi Borsari 46, 44121, Ferrara, Italy 3 Dipartimento di Chimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy 4 Istituto Nazionale di Scienza e Tecnologia dei Materiali, via Giusti 9, 50121, Firenze, Italy * Corresponding Author. [email protected] Abstract In this work we demonstrate the feasibility of hard X-rays operando XAS in photoelectrochemistry. WO3, one of the most studied photoanodes for water splitting and for environmental remediation, is here studied at the W LIII-edge. This guarantees the direct observation of the W 5d band. The material, that is preliminary fully characterized in terms of its photoelectrochemical features, is studied in a three-electrode spectroelectrochemical cell, while X-ray absorption is measured in the X- ray absorption near edge structure (XANES) region. The recording of differential spectra and the monitoring of X-ray absorption at constant energy are used to compensate for the little XANES differences expected in the dark and under visible light illumination, which otherwise risks to be masked by experimental errors and/or after signal manipulation for data analysis. The results point to the filling of the W t2g orbitals under illumination, that is followed by a structural rearrangement that compensates for the accumulation of electrons in the conduction band under open circuit (OC) conditions. Keywords: photoelectrochemistry, spectroelectrochemistry, water splitting, XANES, FEXRAV, tungsten oxide. 1 1. Introduction The need for the efficient exploitation of renewable energy sources calls for the development of suitable energy conversion devices. In particular, solar energy harvesting requires the use of an energy vector capable of storing sunlight energy to compensate its intermittent nature. H2 is the best candidate to this role, and photoelectrochemical water splitting (PEC-WS) guarantees the production of high purity H2. In PEC-WS, a semiconductor immersed in solution and coupled to a counter-electrode is illuminated by solar light. Light absorption by the semiconductor causes the formation of electron/hole pairs, which are separated and can drive two half-reactions thanks to the electrical field generated within the semiconductor at the semiconductor/liquid junction (SCLJ). Quite often, this requires the help of an external applied potential (bias). For n-type semiconductors, the anodic reaction (that proceeds by the transfer of holes to water) occurs at the semiconductor surface while the cathodic reaction is driven at the counter-electrode. In the case of water splitting, the anodic process is the oxygen evolution reaction (OER): - + 2H2O → 4e + 4H + O2 (1) That is a complicated reaction due to the need of exchange 4 electrons for each O2 molecule. In addition, electron-hole recombination, either in the bulk of the semiconductor or at surface defects represents the major parasitic phenomenon limiting the solar-to-hydrogen efficiency. Several methods for better understanding the physical-chemistry of the SCLJ have been proposed so far[1], starting from the analysis of photocurrent transients to time-resolved absorption spectroscopy[2]. X-ray absorption spectroscopy (XAS) represents a powerful tool in that allows determining the oxidation state and the chemical surrounding of an absorbing atom. Moreover, XAS deals with the excitation of core (non-valence) electrons: this is key to make the technique element selective. XAS is a bulk technique, as hard X-rays have a penetration depth of the order of microns. This represents a major drawback in studying interphases, since XAS averages the properties of all absorbing atoms within the X-ray beam. Notwithstanding this, operando XAS in electrochemistry proved to be effective in probing absorption phenomena and in determining the nature of active sites in inner sphere reactions[3][4][5]. In photoelectrochemistry, the use of operando XAS is quite scarce[6], being limited to a poor number of studies, i.e. of hematite photoanodes[7] and of relevant overlayers. In the latter case, the operando experiment involved the detection of any XAS spectral change of the overlayer (IrOx, thus at the Ir LIII edge) while the semiconductor was in the dark or illuminated[8][9]. This limited number of papers is likely due to (i) the limited access to synchrotrons and (ii) to the difficulty in detecting, with significant statistics, the subtle spectral differences between 2 a photoelectrode in the dark and under illumination. In fact, together with the aforementioned limit of surface selectivity (where photogenerated charges might accumulated and be detected), recombination strongly limits the number of long-living photogenerated charges. Indeed, while short- living charges can be detected by ultrafast time resolved XAS, this has never been realized under operative, photoelectrochemical conditions. We foresee two possible solutions. The first one deals with the ad-hoc preparation of a sample with a very high surface-to-bulk ratio. In this way, charges stored at the interface could be better detected. However, this kind of sample likely represents a poor performant photoelectrode and the “operando” conditions are not fully satisfied. The second option consists in carrying out specific XAS techniques for direct comparing “dark” and “light” conditions, making minor changes (in the order of a few percent in the absorption coefficient, µ) well evident above the experimental error. In this work, we implement the second strategy for the operando study of WO3, one of the most studied and promising n-type semiconductors for PEC-WS. We developed two complementary methods: 1) The acquisition of differential spectra : for each X-ray photon energy the acquisition was performed both under visible light and in the dark to minimize possible systematic errors. This ensures that for each energy value the absorption coefficient under light and dark conditions is measured under the same instrumental conditions and can be directly subtracted. 2) The X-ray energy is kept constant, at values corresponding to the highest contrasts between the absorption coefficient of different standards phases. Obviously, this implies to record spectra of standard materials. This approach is similar to the one that we adopted for FEXRAV (fixed energy X-ray absorption spectroscopy)[10–12] where, under operative conditions and during the acquisition of the absorption coefficient, the electrode potential is varied at will. In the case of photoelectrochemical phenomena, this approach can be used to immediately compare the absorption coefficient under light and in the dark, recording the photocurrent and the spectroscopic information in sync, without the need of any data treatment. As model system, we chose WO3, one of the most studied visible absorbing n-type semiconductor to be used as photoanode for water splitting and for wastewater remediation.[13] Moreover, X-ray absorption at the W LIII-edge (10.204 keV) implies the excitation, in the X-ray absorption near edge structure (XANES) region, of 2p to 5d orbitals. This reflects on the possibility of a direct observation of the occupancy of the 5d band of W. In this work, we show that operando XAS for the study of an SCLJ is feasible and that it allows the direct observation of the semiconductor bands under working conditions. 3 2. Experimental 2.1 Preparation of WO3 photoanodes H2WO4 was generated from 2.5 g of Na2WO4 (AlfaAesar) by addition of 20 ml of concentrated HCl (Aldrich), followed by several washings in order to eliminate NaCl. The colloidal suspension of H2WO was obtained by addition of 2 g of oxalic acid (Aldrich) at 60 °C. Transparent nanocrystalline electrodes cast onto well cleaned FTO (Pilkington TEC 7) glass were prepared by sequential spin coating deposition of the H2WO4 precursor prepared by adding 20% w/w Carbowax (Aldrich, 15000– 20000 u) and triton X-100 (Fluka) (1 drop/2 g of colloidal precursor) to the H2WO4/oxalic acid colloid [21]. After each deposition the electrode was heated at 550°C for 30 min in air. Up to six spin coated layers afforded nanocrystalline electrodes having a thickness of ca. 1.5 µm. All electrodes have geometrical active area of 1 cm2. 2.2 Photoelectrochemical and Electrochemical measurements. Linear sweep voltammetry (LSV) curves of the WO3 photoanodes under solar simulated illumination (ABET AM 1.5 G) were recorded with an Autolab PGSTAT 302/N in a three electrode cell containing 0.5 M Na2SO4, at a scan rate of 20 mV/s in the -0.2/1.5 V vs. SCE interval. Shuttered curves were obtained by manually shutting the illumination source. Photovoltage measurements were performed by zero current chronopotentiometry by preconditioning the photoanode at 300 mV vs. SCE for 180 s. The photovoltage relaxation with time, following restoration of the dark conditions was fitted with a biexponential function (1) in to extract the relative time constants −푥⁄푡1 −푥⁄푡2 푦 = 푦0 + 퐴1푒 + 퐴2푒 (1) Incident photon to current efficiency (IPCE) spectra were collected with a previously described apparatus and [14] under constant 1.5 V vs. SCE bias. Uv-Vis spectra were obtained in transmission mode with a JASCO UV-570 spectrophotometer. Cyclic voltammetry was recorded with scan speeds ranging from 10 mV/s to 100 mV/s between - 0.2/1.5 V vs. SCE in the dark. In the following data presentation and discussion all measured potentials vs SCE, were converted to the RHE scale. 4 2.3 Operando XAS. X-ray Absorption Spectra at the W-LIII edge were acquired in the fluorescence mode at the LISA beamline [http://www.esrf.eu/UsersAndScience/Experiments/CRG/BM08] at ESFR (European Synchrotron Radiation Facility), using a Si(311) double crystal monochromator, Pd mirrors with a cut-off energy of 20 keV for the harmonic rejection and a 13-element Ge fluorescence detector.
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